A pressure sensor is utilized to measure a pressure of various circumstances, such as a pressure of a combustion gas (combustion pressure) in an internal combustion engine.
(FIRST PRIOR ART) A prior art pressure sensor shown in FIG. 13 is provided with a housing 20, a diaphragm 102, a force transmitting member 52 and a sensor element 54. The diaphragm 102 partitions an interior and an exterior of the housing 20. The force transmitting member 52 is provided within the housing 20 and moves downwardly in the figure, when a pressure at the exterior of the housing 20 is exerted upon a top surface 102a of the diaphragm 102. The sensor element 54 is provided within the housing 20 and changes output value in accordance with a force exerted thereupon. A force due to the pressure at the exterior of the housing 20 is exerted upon the sensor element 54 via the diaphragm 102 and the force transmitting member 52.
When the pressure sensor is located within a combustion chamber of an internal combustion engine and a pressure of a combustion gas operates upon the diaphragm 102, a force due to the combustion pressure is exerted upon the sensor element 54. Consequently, it should be possible to measure the combustion pressure from the output value of the sensor element 54.
The pressure operating upon the diaphragm 102 is not constant, but instead varies in accordance with a crank angle of the internal combustion engine. The pressure operating upon the diaphragm 102 is largest during an explosion process of the internal combustion engine, and smallest during an admission process. During the explosion process, high pressure of the combustion gas operates upon the diaphragm 102 and simultaneously high temperature of the combustion gas also operates thereupon.
When a high temperature fluid makes contact with the diaphragm 102, thermal expansion of the outer surface 102a (the side making contact with the high temperature fluid) of the diaphragm 102 occurs, and a shape of the diaphragm 102 is changed due to temperature increase as shown in FIG. 14, that is, a center region of the diaphragm 102 is moved upwardly. As a result, a contacting face between the diaphragm 102 and the force transmitting member 52 moves (drift) upwardly from a predetermined reference position. Consequently, a posterior end face (a face contacting with the sensor element 54) of the force transmitting member 52 shown in FIG. 13 moves upwardly.
As a result, in the case where the combustion gas pressure is exerted upon the diaphragm 102, the combustion gas heat causes upward movements of the diaphragm 102 and the force transmitting member 52, and the sensor element 54 outputs a value smaller than an output value corresponding to the actual pressure. An output error of the sensor element 54 is thus created. For example, as shown in FIG. 15, with the horizontal axis showing the crank angle of the internal combustion engine and the vertical axis showing the output values of the sensor element 54, a graph C obtained from measurement by the pressure sensor shown in FIG. 13 has lower values than a graph B corresponding to actual pressure variations. The pressure thus detected is excessively low.
(SECOND PRIOR ART) In a pressure sensor as shown in FIG. 16, a central region 112 of a diaphragm 110 is shifted downwardly with respect to a surrounding region of the diaphragm 10. In this pressure sensor, even if a thermal expansion of a top surface 110a (the side making contact with the hot combustion gas) of the diaphragm 110 occurs, a contacting face between the diaphragm 110 and the force transmitting member 52 is prevented from moving upwardly, and the output error of the sensor element 54 is lower than in the case of the first prior art.
The detail of the second prior art is disclosed in Japanese Laid Open Patent Publication (TOKKAI-HEI) 7-19981 (specifically in FIG. 1).
However, there is a problem that the output error of the sensor element 54 still cannot be sufficiently reduced by the pressure sensor of the second prior art shown in FIG. 16. In this pressure sensor, the diaphragm 110 and the force transmitting member 52 can be prevented from moving upwardly when the high temperature combustion gas makes contact with the diaphragm 110. However, when the high temperature combustion gas makes contact with the diaphragm 110, a thermal expansion of downwardly-inclined portions 114 occurs. As a result, a contacting face between the diaphragm 110 and the force transmitting member 52 moves (drifts) downwardly with respect to the housing 20. Consequently, a posterior end face (a face contacting with the sensor element 54) of the force transmitting member 52 also moves downwardly.
As a result, in the case where the hot combustion gas pressure is exerted upon the diaphragm 110, the combustion gas heat causes downwards movements of the diaphragm 110 and the force transmitting member 52, and the sensor element 54 outputs a value greater than an output value corresponding to the actual pressure. An output error of the sensor element 54 is thus created. For example, as shown in FIG. 15, with the horizontal axis showing the crank angle of the internal combustion engine and the vertical axis showing the output values of the sensor element 54, a graph A obtained from measurement by the pressure sensor shown in FIG. 16 has greater values than the graph B corresponding to the actual pressure variations. The pressure thus detected is excessively high.